Protein kinases regulate various cellular responses to changing environmental conditions. Protein kinases fall into two general classes: those protein kinases that transfer phosphate to serine or threonine and those proteins that transfer phosphate to tyrosine (Krebs and Beavo, Annu. Rev. Blochem 48: 923-959 (1979)). A few protein kinases, such as weel, now appear to be capable of phosphorylating both ser/threonine and tyrosine (Lindberg et al., Trends Biochem Sci 17: 114-119 (1992)). Phosphorylation is of particular significance in controlling mitogenesis and cellular differentiation. Receptors for a number of polypeptide growth factors are transmembrane tyrosine kinases (Yarden and Ullrich, Annu. Rev. Biochem 57: 443-478 (1988)), which in turn stimulate serine/threonine kinases such as protein kinase C, MAP kinase and p74.sup.raf (Hunter et al., Nature 311: 480-483 (1984); Morrison et al., Cell 58: 649-657 (1989); Rossomondo et al., Proc. Natl. Acad. Sci. USA 86: 6940-6943 (1989)) .
Protein kinases, and especially the overexpression thereof, have been found to be linked to hyperproliferation of cells and metastasis. Many protein kinases were first identified as the products of oncogenes and still constitute the largest family of known oncogenes. Lindberg and Hunter, Mol. and Cell. Biol., 10(11): 6316-6324 (1990).
Mutations of genes encoding members of the protein kinase family which are involved in the regulation of neuroblastic proliferation, differentiation and survival play a role in the etiology of human central nervous system tumors. Thus, it is highly desirable to gain a greater understanding of this class of proteins, as well as to use such greater understanding to limit or inhibit the effects that these proteins have on cellular hyperproliferation.